A heat resisting structure wherein stainless steel corrugated sheets having alternating ridges and grooves formed by folding the sheet continuously and flat stainless steel sheets, etc. are joined together with nickel(Ni)-base brazing filler metal to form honeycomb-form structure is conventionally used. In such a conventional heat resisting structure, stainless steel corrugated sheets, flat sheets, etc. which are highly resistant to corrosion and heat are used as base metal and nickel(Ni)-base brazing filler metal which is also highly resistant to corrosion and heat is used to withstand a high environmental temperature. The heat resisting structure is formed into a roll-form by rolling up the corrugated sheet, flat sheet, etc. and joining together with nickel(Ni)-base brazing filler metal or formed into a laminated block by stacking the materials into many layers and joining together, and thus formed honeycomb-form structure is utilized as a catalyst holding body and for other various uses.
Now, in relation to the corrugated sheet, flat sheet, brazing filler metal, etc. of the heat resisting structure, the prior art will be examined in detail. First, as to the corrugated sheet, flat sheet, etc. constituting the base metal, those of austenitic stainless steel are used. Recently, those of ferritic stainless steel also are in use. As to the brazing filler metal, nickel(Ni)-base brazing filler metal is used generally by applying it to the entire surface. The brazing filler metal is provided over the entire surface between the corrugated sheet, flat sheet, etc. constituting the base metal by, for example, coating the entire surface, placing foil-like brazing filler metal between the sheets over the entire surface, or using a brazing sheet coated with brazing filler metal over the entire surfaces. Composition of brazing filler metal currently in use is, by weight, 0.1% C maximum, 7.0 to 8.0% silicon Si, 18.0% to 9.0% chromium Cr, 1.0 to 1.5% boron B, and the remainder is nickel Ni.
However, the following problems have been pointed out concerning the above-mentioned conventional heat resisting structure.
First, the heat resisting structure employing austenitic stainless steel corrugated sheets, flat sheets, etc. is excellent in oxidation resistance as well as high-temperature strength to withstand a high environmental temperature during the use. However, this heat resisting structure, when stress is applied during use at a high environmental temperature, sometimes form stress corrosion cracks in the corrugated sheets, flat sheets, etc. constituting the base metal, in some cases carbide is precipitated on grain boundaries of the austenitic portion of the base metal, where intergranular corrosion cracking occurs, and thus durability of the structure at a high environmental temperature is affected. In addition, there is a problem in cost due to the expensiveness of the material. To solve the problems in the heat resisting structure of austenitic stainless steel mentioned above, heat resisting structure wherein ferritic stainless steel corrugated sheets, flat sheets, etc. are used as the base metal has been developed.
Secondly, in the heat resisting structure employing stainless steel, particularly ferritic stainless steel corrugated sheets, flat sheets, etc., the state of elements of the alloy, so-called phase, of the corrugated sheets, flat sheets, etc. which are the base metal is different from that of nickel(Ni)-base brazing filler metal. Therefore, when nickel(Ni)-base brazing filler metal diffuses into the base metal owing to brazing for joining, the phase tends to change in the vicinity of the boundary where the base metal is joined, causing phase transition. (For example, transition to gamma phase. Hereinafter, the same applies.) Therefore, use at a high environmental temperature is sometimes affected adversely. More specifically, when such conventional heat resisting structure is used at a high environmental temperature, chromium Cr contained in nickel(Ni)-base brazing filler metal is segregated in the vicinity of the boundary where the base metal of, for example, ferritic stainless steel is joined by brazing, making the chromium Cr concentration very high at the portion of the metal phase, and the portion of low chromium Cr concentration is prone to oxidation. Further, when the conventional heat resisting structure is used at a high environmental temperature, carbon C contained in the nickel(Ni)-base brazing filler metal is sometimes precipitated as carbide on the grain boundaries in the vicinity of the boundary where the base metal is joined by brazing. The precipitated material reduces the strength lower than the toughness. Therefore, stress corrosion cracking, intergranular corrosion cracking, intergranular separation, etc tend to occur in the austenitized portion in a similar manner to that mentioned above. (For example, transition to gamma phase. Hereinafter, the same applies.)
Thirdly, because the phase of the stainless steel, for example, ferritic stainless steel corrugated sheets, flat sheets, etc. is different from the phase of nickel(Ni)-base brazing filler metal, there is a difference in the coefficient of thermal expansion between the two. Consequently, repeated use of the heat resisting structure sometimes generates stress cracks between them. Particularly, since there is a great difference in the coefficient of thermal expansion between the austenitized portion and the base metal, cracks tend to occur there as mentioned above.
Fourthly, due to above-mentioned reasons, the problem that a conventional heat resisting structure is poor in oxidation resistance and durability when used at a high environmental temperature has been pointed out. Furthermore, in such a heat resisting structure, brazing filler metal which causes the above-mentioned problem is arranged in large quantities over the entire surface between the corrugated sheets and flat sheets constituting the base metal, therefore oxidation rate is large, oxidation resistance is poor, and durability is also poor.
Fifthly, when such conventional heat resisting structure is used, for example, for a catalytic converter to treat exhaust gas of an automobile engine as a catalyst holding body to which catalyst is made adhered, a problem has been pointed out in treatment efficiency of the catalyst. In detail, since brazing filler metal made of nickel(Ni)-base brazing filler metal is used in quantities over the entire surface as described previously, adhesion of alumina wash-coat to be applied as primer between the corrugated sheets, flat sheets, etc. constituting the base metal and the catalyst to the base metal is poor, and as a result, adhesion of the catalyst to the base metal is sometimes impaired. Even if the catalyst adheres normally, the catalyst dissolves into the brazing filler metal as a solid solution, thereby lowering the catalyst density at the surface of the base metal. As described above, when the conventional heat resisting structure is used as a catalyst holding body, the catalyst is difficult to adhere and tends to dissolve into the brazing filler metal as solid solution, therefore, treatment efficiency of the catalyst becomes low and exhaust gas treatment efficiency becomes poor.
Such problems have been pointed out concerning the conventional heat resisting structure.